1. Field Of The Invention
The present invention provides a method whereby fractures may be created from a highly deviated or horizontal well bore penetrating a formation in a direction other than the direction of the least principal horizontal stress utilizing minimum breakdown pressures while obviating choking caused by fracture reorientation.
2. Description Of The Prior Art
In the production of hydrocarbons or other fluids from subterranean rock formations penetrated by well bores, a commonly used technique for stimulating such production is to create and extend fractures in the formations. Most often, the fractures are created by applying hydraulic pressure to the subterranean formations from the well bores penetrating them. That is, a fracturing fluid is pumped through the well bore and into a formation to be fractured at a rate such that the resultant hydraulic pressure exerted on the formation causes one or more fractures to be created therein. The fractures are extended by continued pumping and the fractures are either propped open by a propping agent, e.g., sand, deposited therein or the fracture faces are etched by a reactive fluid such as an acid whereby hydrocarbons contained in the formation readily flow through the fractures into the well bore.
The term "subterranean formation" is used herein to mean an entire subterranean rock formation bounded by formations formed of dissimilar rock materials or a hydrocarbon containing zone disposed within a larger rock formation.
Most fractures formed in formations by applying hydraulic pressure thereto lie in substantially vertical planes and extend outwardly from the well bore in a direction at right angles to the in situ least principal stress in the formation. When fractures are created from a substantially vertical well bore penetrating the formation, only two vertical fracture wings generally are produced which extend from opposite sides of the well bore in a direction at right angles or perpendicularly to the in situ least principal stress in the formation. Such fracture arrangement represents the ultimate fluid communication between the well and the fracture. Normally, at the end of the fracturing treatment, maximum fracture height and width will be along the vertical well bore walls. The high costs associated with drilling operations has resulted in a desire to enhance the production capability of a well through multiple parallel fracturing of a formation. As a result hydrocarbon reservoirs are now being developed through the use of horizontal wells from which a plurality of spaced parallel fractures can be created. Fractures induced in a subterranean formation from a well bore also are substantially vertical and are in a direction perpendicular to the least principal horizontal stress. A fracture induced in a horizontal well, however, does not extend in a plane aligned with the well bore as in a vertical well unless the well is drilled in a direction perpendicular to the least principal horizontal stress in the formation. For other horizontal well directions, the initial fracture geometry is dependent on the well deviation from the least principal horizontal stress. Not far from the well bore, the fracture reorients regardless of its initial direction to become perpendicular to the direction of the least principal horizontal stress. The reorientation of the fracture results in a fracture width at the well bore which generally is less than that achieved by fractures created from well bores drilled in the direction of the least principal horizontal stress in the formation. Further, during the reorientation, the fracture is extended under both shearing and tensile modes which result in rough surfaced fracture faces which increase fluid friction pressures near the well bore thereby increasing the operating pressures required to propagate the fracture.
It would be desirable to provide a method of fracturing subterranean formations penetrated by horizontal well bore in a direction other than the direction of least principal horizontal stress which avoids or minimizes the problems of reorientation of fractures.